1. Field of the Invention
The present invention relates in general to a light-emitting material and an organic electroluminescent device using the same and, more particularly, to a light-emitting material including two pyrenes bonded together via different link rings and an organic electroluminescent device using the same.
2. Description of the Related Art
A conventional organic electroluminescent device (OELD) is a multi-layer stacked structure including a substrate, an anode, a cathode, a hole injection layer, a hole transport layer, an electron transport layer, an electron injection layer and an emissive layer. The anode, the hole injection layer, the hole transport layer, the emissive layer, the electron transport layer, the electron injection layer and the cathode are disposed on the substrate orderly from bottom to top. The emissive layer includes a host-dopant system, in which a few dopants are doped in host material. The way to define a fluorescent host-dopant system and a phosphorescent host-dopant system are illustrated as follows.
When a voltage is applied to the cathode and the anode, electrons are injected into the emissive layer from the cathode through the electron injection layer and the electron transport layer. Holes are injected into the emissive layer from the anode through the hole injection layer and the hole transport layer. After the electrons and the holes combine in the emissive layer, the host material is excited from a ground state to an excited state. Because the host material in the excited state is unstable, the host material certainly returns to the ground state and transfer energy to the dopants.
When the dopants receive the energy and are excited from the ground state to the excited state, singlet excitons and triplet excitons are generated by the dopants. In both the fluorescent dopants and the phosphorescent dopants, due to the distribution ratio of the electron spin state, the probability of forming the triplet excitons and the singlet excitons is approximately 3:1.
Electroluminescence occurs in the organic electroluminescent device while the singlet excitons or the triplet excitons return to the ground state by releasing photons. In the fluorescent host-dopant system, only the singlet excitons emit visible fluorescence when returning to the ground state. In the phosphorescent host-dopant system, when returning to the ground state, the triplet excitons emit visible phosphorescence and the singlet excitons emit light which can be transferred to phosphorescence through internal system crossing (ISC).
The conventional emissive layer includes pyrenes. The pyrenes are planar structures, easily resulting in π-π stacking among the pyrenes. As a result, the light color and the luminance efficiency of the organic electroluminescent device using pyrenes are affected seriously. Therefore, aryl group and t-butyl group are mostly induced in the conventional emissive materials, for acting as substituted group of pyrenes. Accordingly, the chemical structure of pyrenes having substituted group is three-dimensional, so as to decrease the effect of π-π stacking on the light color. Although the three-dimensional properties of pyrenes become better due to the existence of substituted groups, the substituted groups beside pyrenes lower the light color and the luminance efficiency of the organic electroluminescent device using pyrenes with the substituted groups.